DRC

A Dynamic Reaction Coordinate calculation [29] is to be run. By default, total energy is conserved, so that as the 'reaction' proceeds in time, energy is transferred between kinetic and potential forms. See
Dynamic and Intrinsic Reaction Coordinates for more details.

By default, steps of 0.1fs are used. This default can be changed using keywords:

Use LARGE to control print of internal coordinates and Cartesian coordinates.

A useful keyword for limiting the DRC is BIGCYCLES=n. BIGCYCLES=n will run until
n complete oscillations have occurred. Normal modes of
vibration can be generated using BIGCYCLES=1 and IRC=n, where n is the n't normal mode of interest.
Such a normal mode would involve one complete oscillation, from equilibrium to
turning point back to equilibrium, then up the other side of the potential well
to the other turning point, then back to the equilibrium position, at which
point the DRC is stopped.

Most of the trajectories mapped out by BIGCYCLES=1 IRC=n DRC are
anharmonic, that is, they reflect the asymmetry of the Morse potential.
They are, however, accurate within the semiempirical approximation used.
Thus if a vibrational frequency is m cm-1, the duration of one
complete vibration will be 1/(m.c) seconds. For example, if a
normal mode has an energy (frequency) of 1000 cm-1, each complete
vibration would take 33.4 femtoseconds.